1. Field Of The Invention
The present invention relates to optical parametric oscillators. More particularly, the present invention relates to a synchronously pumped optical parametric oscillator for generating high-power, broadly tunable pulses with subpicosecond duration.
2. The Prior Art
Optical parametric oscillators (OPO) have been known for many years. Recently, cw mode-locked OPO's have been demonstrated. The first system was demonstrated by E. S Wachtman, D. C. Edelstein, and C. L. Tang, Opt. Lett. 15, 136 (1990) and used a CPM laser as the pump source. The nonlinear crystal was KTiOPO.sub.4 (KTP) which has the highest nonlinear coefficient of any suitable crystal and thus the greatest chance of reaching threshold. To obtain sufficient pump power the KTP crystal was placed both inside an OPO cavity and inside the CPM cavity. This increased the pump power by two orders of magnitude and greatly increased the complexity of the system. U.S. Pat. No. 5,017,806 discloses much the same system as described in the Edelstein et al. article. The wavelength of the OPO is adjusted by rotation of the crystal.
Recently, cw mode-locked Ti:sapphire lasers have been demonstrated. These lasers are capable of reaching the same peak powers outside the cavity as inside the CPM laser and are thus useful as pump sources. The average power output of a low mode-looked Ti:sapphire laser is typically 1 to 2 Watts, with a pulse duration of about 100 femtoseconds. KTP OPO's using this pump source have been demonstrated by several groups. Q. Fu, G. Mak, and H. M. van Driel, Opt. Lett. 17, 1006 (1992) used 110 femtosecond pump pulses and angle-tuned KTP to produce tunable sub-picosecond pulses. W. S. Pelough, P. E. Powers, and C. L. Tang, Opt. Lett. 17, 1070 (1992) obtained similar results using 125 femtosecond pump pulses and angle tuned KTP.
A. Nebel, U. Socha, and R. Beigang, in Digest of Conference on Ultrafast Phenomena VIII, (E.N.S.T.A. Paris 1992) paper ThC1, used a 1.4 picosecond Ti:sapphire laser and a 90.degree. phase matched KTP crystal to produce tunable picosecond pulses. This crystal configuration has several advantages over the angle tuned system, in that the nonlinear coefficient is maximum and there is no walkoff. Both of these attributes contribute to a more efficient OPO with potentially higher output power. In addition because of the lack of walkoff (between the pump beam and the OPO beam) the alignment procedure can be made significantly simpler than in the angle tuned systems. One drawback, however, is that the crystal can no longer be rotated to tune the output. The output of the pump laser had to be tuned in order to tune the output.
The angle-tuned KTP systems have several drawbacks. When the OPO is tuned, the crystal is rotated. The cavity length must then be adjusted and the cavity must be realigned after rotation of the crystal. To obtain the widest possible tuning range, multiple crystals may be necessary due to the AR coatings required. The alignment is complicated because the pump beam and OPO beam are not collinear. This walkoff requires the pump beam to be brought in at a specified angle to the OPO beam. The beams must also be focussed to less than 50 microns and cross in the nonlinear crystal.
Using 90.degree. phase matched KTP removes the walkoff problem and simplifies the alignment. The pump laser must be tuned, however, and then the dispersion of the pump laser must be adjusted. Then the OPO cavity length needs to be reoptimized. In addition, multiple sets of optics are required for the pump laser. Even with these additional optics sets, the tuning range of the OPO is limited, and a region of non-coverage exists between 1.5 and 2.2 microns. In addition, the output power will drop severely at longer wavelengths due to the drop in pump power.
The only nonlinear crystal other than KTP which has been successfully demonstrated in a cw synchronously pumped OPO is LiB.sub.3 O.sub.5 (LBO). A. Robertson, G. P. A. Malcolm, M. Ebrahimzadeh, and A. I. Ferguson, Postdeadline paper CPD15 from CLEO 1992, demonstrated an LBO OPO utilizing a 2.5 picosecond pulse at 524 nm from a frequency-doubled, mode-locked Nd:YLF laser as the pump source. They generated picosecond pulses using 90.degree. phase-matched LBO and used temperature tuning to tune the output. The crystal length was 12 mm and the authors state that it should be possible to generate femtosecond pulses by using a pulse-compressed pump source.
The 90.degree. phase matched LBO crystal has all the advantages of 90.degree. phase matched KTP with the added advantage of temperature tuning. In particular, only the temperature and the cavity length need to be adjusted to tune the OPO. In the case of angle tuned OPO's, the cavity must also be realigned slightly if the OPO is tuned over any significant wavelength difference. Unfortunately the change in index of LBO with respect to temperature is not a well known quantity yet. The only published data is in S. P. Velsko, M. Webb, L. Davis and C. Huang, IEEE J. Quantum Electron. 27, 82 (1991); and S. Lin, J. Y. Huang, J. Ling, C. Chen, and Y. R. Shen, Appl. Phys. Lett. 59, 2805 (1991). This data from these papers is contradictory and does not accurately predict the observed performance. Whether the LBO crystal would work with a Ti:sapphire pump at 800 nm at a reasonable temperature (less than 200.degree. C.) was not known and cannot be predicted from the literature. This is why Robertson teaches to compress the 524 nm source as opposed to switching to a Ti:sapphire pump. It is widely believed that the crystal temperature would be too hot for 800 nm pumping.
It is an object of this invention to provide an OPO which can generate broadly tunable pulses with sub-picosecond duration.
A further object of the invention is to provide an OPO which can produce the highest power and shortest pulses from a simple and reliable system.
Yet another object of the present invention is to provide an OPO which overcomes the drawbacks of prior-art OPO systems.
Another object of the invention is to provide an OPO which is easy to align and requires minimum adjustment during tuning.
Still a further object of the invention is to provided an OPO which produces the highest power and shortest pulses over a tuning range larger than that available in present OPO systems.